This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-217017, filed Jul. 17, 2001, the entire contents of which are incorporated by reference.
1. Field of the Invention
This invention relates to a process for hydrogenating carbon monoxide. More specifically, this invention relates to a process for producing synthetic fuels of low environmental impact from synthesis gas. In one aspect, this invention concerns a catalyst for use in the hydrogenation of carbon monoxide.
2. Description of the Related Art
Various useful organic chemicals have been produced from carbon resources like petroleum, coal, natural gas, and biomass in the following manner. Firstly, a synthesis gas, a mixture of carbon monoxide and hydrogen, is produced through a reforming reaction or a coal gasification. The synthesis gas is then allowed to react on specific catalysts at high temperature and at high pressure, thus converted to hydrocarbons or oxygenates.
These organic chemicals thus obtained will suppress the emission of toxic substances when used as fuel, since they contain little sulfur and nitrogen compounds owing to their distinctive manufacturing processes. In particular, methanol, which is mostly produced from synthesis gases and used as an alternative fuel or a gasoline additive, has recently received much attention as a hydrogen source for the fuel cell. In the stream of rising environmental-conscious, an improved manufacturing method with higher productivity is desired.
In the reaction of synthesis gases, catalysts including metals such as Cu, Fe, and Co are generally used. Typical review articles are in the text xe2x80x9cStudies in surface science and catalysis, vol. 81, NATURAL GAS CONVERSIONxe2x80x9d, H. E. Curry-Hyde, R. F. Howe, Elsevier (1994).
While Cu is widely used for the production of methanol, it is also known in the art that Rh, Pd, Ir and Pt have considerable activity for the alcohol synthesis and that the activity may be promoted by the addition of alkali metal, alkali earth metal, and rare earth elements.
For instance, U.S. Pat. No. 4,119,656 discloses that methanol is selectively produced on a silica supported Pd (Pd/SiO2) catalyst. U.S. Pat. Nos. 4,289,709 and 4,289,710 disclose the promoting effect of Li, Mg, Sr, Ba, Mo, and Ca in the methanol synthesis on a Pd/SiO2 catalyst. A. Gotti and R. Prins in Journal of Catalysis, 175, 302-311(1998) have mentioned the promoting effect of Ca and La on activity and selectivity in the methanol synthesis.
Among these catalysts, Cu catalysts are commercially used for the methanol synthesis because of their low cost and availability, in spite of the drawbacks of requiring high temperature and high pressure conditions. However, Cu catalysts are easily poisoned by various chemical substances in feed gases, particularly by a trace amount of sulfur compounds such as hydrogen sulfide. To avoid this sulfur poisoning, sulfur compounds must be reduced far less than 1 ppb through a desulfurization facility before the reforming or hydrogenation reaction process. Consequently, the use of Cu catalysts make the manufacturing process complicated and expensive.
Nevertheless, misoperation or accidents leading to contamination of sulfur compounds might damage the catalysts.
On this sulfur poisoning, Jpn. Pat. Appln. KOKAI Publication Nos. 55-139324 and 55-139325 disclose a production process of hydrocarbons with sulfur tolerant catalysts that consist essentially of the metal, oxide or sulfide of Mo, W, Re, Ru, Ni, Pd, Rh, Os, Ir or P, and alkali metal or alkaline earth metal. In these applications, it is noted that a catalyst consisting of MoO3, K2O and carborundum shows no remarkable change in activity and gaseous alkene selectivity even when a synthesis gas contains 20 ppm of hydrogen sulfide.
U.S. Pat. No. 4,749,724 discloses that C1-C4 alcohols are produced from a synthesis gas containing hydrogen sulfide at high pressures around 10 MPa on a sulfided Mo, W or Re catalyst added with alkali metal or alkaline earth metal.
As mentioned above, Cu catalysts are deactivated by sulfur compounds, so that the content of sulfur compounds must be lowered far less than 1 ppb by means of upstream desulfurization units. On the other hand, aforementioned sulfide catalysts containing Mo, W or Re require high pressure conditions to achieve proper activity and selectivity.
The object of the present invention is to provide a method for hydrogenating carbon monoxide with high productivity under mild conditions and with a simple manufacturing process. It is another object of the present invention to provide sulfide catalysts with high durability, especially excellent sulfur tolerance in the production of synthetic fuels.
According to one aspect of the present invention, there is provided a method for producing synthetic fuels by hydrogenating carbon monoxide comprising contacting a feed gas containing carbon monoxide and hydrogen with a supported metal sulfide catalyst comprising Pd and at least one promoter selected from the group consisting of alkali metal, alkaline earth metal, and rare earth elements.
According to another aspect of the present invention, there is provided a method for producing synthetic fuels by hydrogenating carbon monoxide comprising contacting a feed gas containing carbon monoxide and hydrogen with a catalyst including a solid acid and a supported metal sulfide comprising Pd and at least one promoter selected from the group consisting of alkali metal, alkaline earth metal, and rare earth elements.
The catalyst used in the practice of the invention is a supported metal sulfide comprising Pd and at least one promoter selected from the group consisting of alkali metal, alkaline earth metal, and rare earth elements. The metal sulfide catalyst may be prepared by sulfiding of corresponding Pd compounds.
The sulfiding may be carried out by contacting the Pd compounds with sulfur compounds such as lithium sulfide, sodium sulfide, potassium sulfide, ammonium sulfide, hydrogen sulfide, and thiophene with gradually increasing the temperature up to 150-250xc2x0 C. and then to a predetermined operation temperature where temperature is maintained for 1-4 hours. The sulfiding may also be carried out by treating the Pd compounds with sulfur compounds contained in high concentrations in a feed gas during the hydrogenation reaction.
The exemplary Pd compounds include metallic palladium such as palladium black, palladium carbon, and palladium on calcium carbonate, palladium complexes or salts such as ammonium tetrachloropalladate (NH4)2PdCl4, tetraamminepalladium nitrate Pd(NH3)4(NO3)2, tetraamminepalladium chloride Pd(NH3)4Cl2, tetraamminepalladium bromide Pd(NH3)4Br2, diamminedichloropalladium PdCl2(NH3)2, diamminedinitropalladium Pd(NH3)2(NO2)2, palladium acetate Pd(CH3COO)2, palladium oxide PdO, palladium cyanide Pd(CN)2, palladium chloride PdCl2, palladium bromide PdBr2, palladium iodide PdI2, palladium nitrate Pd(NO3)2, palladium hydroxide Pd(OH)2, palladium sulfate PdSO4, palladium sulfides PdS, PdS2, bis(acetylacetonato)palladium Pd(C5H7O2)2, bis(ethylenediamine)palladium chloride Pd(C2H8N2)2Cl2, tetrakis(triphenylphosphine)palladium Pd(PPh3)4, potassium tetracyanopalladium K2Pd(CN)4, lithium tetrachloropalladate Li2PdCl4, and calcium tetrachloropalladate CaPdCl4.
The exemplary promoter selected from the group consisting of alkali metal, alkaline earth metal, and rare earth elements include Na, K, Ca, Mg, La, Th, etc. These materials may be used alone or in combination thereof. The promoter may be contained originally in the Pd compounds or supports, or added afterwards. A preferred amount of the promoter is represented by the promoter/Pd molar ratio of 0.01 to 10. The promoter less than 0.01 mol/mol-Pd has little effects on the catalytic activity, while the promoter more than 10 mol/mol-Pd might adversely affect the activity. 0.1 to 1 mol/mol-Pd of the promoter is more preferable. When the promoter is added afterwards, its chlorides, bromides, iodides, oxides, nitrates, phosphates, sulfates, ammonium salts, acetic salts, carbonyls, or chelates may be loaded simultaneously or sequentially with the Pd compound on the support.
The exemplary support materials include inorganic oxides such as silica, alumina, fluorinated alumina, boria, calcia, magnesia, titania, zirconia, silica-alumina, alumina-magnesia, alumina-boria, alumina-zirconia, silica-calcia, silicoalumino phosphate, zeolite, and rare earth metal oxides, clay minerals such as montmorillonite, kaolin, halloysite, bentonite, attapulgite, kaolinite, and nacrite, and carbon. These materials may be used alone or in combination thereof. While any number of materials can serve as a support, magnesia, silica, and calcia are preferred. Metal loading on such supports can improve the activity per unit mass of the loaded metal. The support have a surface area preferably larger than 10 m2/g, and more preferably 100 m2/g. The support having a surface area more than 100 m2/g will provide higher dispersion of metals, eventually leading to higher yields of synthetic fuels. The support may contain nonmetallic elements such as boron and phosphorus.
In preparation of supported catalysts, the support may be impregnated by techniques known as the wet, dry, and vacuum impregnations and the ion exchange method.
The preferred amount of loaded Pd depends on the property of the support and cannot be inclusively determined; preferably it may be 1-30 mass %, more preferably 1-10 mass % of the catalyst. When this amount is less than 1 mass %, the catalyst activity should be lower. On the other hand, when the amount is greater than the above value, the loaded Pd might be agglomerated, so that its activity per unit mass of Pd might be lower.
The sulfide catalyst in the present invention can be used in combination with solid acids. The solid acids include oxides such as alumina, alumina-silica, alumina-boria, alumina-magnesia, and silica-magnesia, zeolites such as X type, Y type, MFI type, and mordenite, and clay minerals such as montmorillonite. xcex3-alumina is most preferred. These solid acids can be used as supports or mixtures with the sulfide catalyst.
The composite catalyst of the solid acid and the metal sulfide enables the direct synthesis of dimethyl ether (DME) from synthesis gas. DME, a promising next-generation clean diesel fuel, is presently produced with a two-step process: methanol synthesis and following dehydration reaction.
In the present invention, the feed gas containing carbon monoxide and hydrogen is flown over the sulfide catalyst and converted into synthetic fuels such as methanol. When the composite catalyst is used, DME can be produced.
The molar ratio of hydrogen to carbon monoxide (H2/CO) in the feed gas is preferably in the range from 1 to 5, more preferably from 1 to 3. This is because (1) the H2/CO molar ratio in the methanol synthesis reaction (CO+2H2=CH3OH) is 2, and (2) the H2/CO molar ratio in synthesis gases produced from the reforming of natural gas is usually greater than unity, in most cases with excessive hydrogen.
The Pd sulfide catalyst in the present invention may contain metals such as Ti, V, Mn, Fe, Co, Zr, Mo, Ru, and Rh, unless they lessen the effect of the present invention. These materials may be used at the amount from 0.1 to 100 parts by mass of Pd sulfide.
The feed gas may contain sulfur compounds in addition to carbon monoxide and hydrogen. The content of the sulfur compounds is preferably 1-10,000 ppm, more preferably 100-2,500 ppm, most preferably 100-500 ppm.
The product yield is also a function of temperature and pressure. The temperature range is preferably between 100 and 400xc2x0 C. more preferably between 250 and 350xc2x0 C. The pressure is preferably between 0.1 and 10 MPa, more preferably between 1 and 7 MPa.
According to the present invention, a feed gas containing carbon monoxide and hydrogen is allowed to react on the specific catalyst, so that we can obtain higher activity and selectivity under lower pressure conditions. Besides this, a simple or no desulfurization unit is required to treat the feed gas because of the excellent sulfur tolerance of the inventive catalyst. This will simplify the manufacturing process of synthetic fuels.
The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
The present invention is illustrated in more detail by reference to the following examples wherein, unless otherwise indicated, all percentages and ratios are by mass. In the examples, the reaction conditions are as follows:
Reactor system: a high-pressure fixed-bed flow reactor
Synthesis gas composition: 33% CO/62% H2/5% Ar